Process for producing insulating iron oxide coatings



Feb 27,1951 P. L. SCHMIDT ETAL 2,543,710

PROCESS FOR PRoDucING INSULATING IRON OXIDE COATINGS wlNEssEs; Pfrcenyep l, L 5 hlNYSgORSd au C ml an Lawrence E HNL Y .A W

Feb. 27, 1951 P. l.. sc HMlDT ETAL.

PRocEss FOR PRoDucING INSULA'J'ING IRON oxIDE coATINGs 2 Sheets-Sheet 2 Filed Jan. 15, 1948 NSY NNN

INVENTORS Paul ..Schmidf and WITNESSES:

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Patented Feb. 427, 1951 PROCESS FOR PRODUCING INSULATING IRON OXIDE COATINGS Paul L. Schmidt, Perrysville, and Lawrence B. Hill, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 15, 1948, Serial No. 2,348

This invention relates to the heat treating of ferrous metal in selected atmospheres under conditions so as to produce stable insulating oxide. surface films.

When the surfaces of ferrous metal are oxif 2 Claims. (Cl. 14s-6.35)

dized, a variety of iron oxides may be formed depending on the temperature and the atmosphere to which the ferrous metal is subjected. The properties of such iron oxide films or coatings vary considerably. Iron oxides produced under some conditions are relatively unstable and below certain temperatures they decompose producing non-insulating surface coatings. In particular. it has been found that when a ferrous metal is oxidized above a temperature of 570 C. that theoxide formed is substantially FeO and when cooled below 570 C. this oxide decomposes to produce Fes04 and Fe. The resulting iilm contains substantial amounts of free iron and the film is electrically conductive. Under some conditions, the red iron oxide Feroz is formed and this oxide is objectionable under all conditions bl'cause it is loose and promotes rusting and has no desirable electrically insulating properties.

It has been discovered that. under certain conditions of temperature and atmosphere, films consisting principally of FeaOi may be produced on ferrous metal surfaces and thatsuch films have extremely high ohmic resistance and will adhere tenaciously to the ferrous metal. 'Ihese films are stable and even protect the surface oi the metal from rusting for long periods of time under the most adverse outdoor conditions.

The object of this invention is to provide a process for treating ferrous metal for consistently producing thereon highly resistant, stable films of iron oxide having the formula Fea04.

The object of this invention is to provide for applying-"to ferrous metal surfaces stable, insulating oxide surface lms having high ohmic resistance and other desirable properties.

A further object of this invention is to provide for annealing ferrous metal under such conditions as to eliminate undesirable surface oxides while simultaneously stress relieving the metal, and thereafter to produce on the ferrous metal stable, insulating oxide surface films of high ohmic resistance.

Other objects of the invention will in part be obvious and will in part appear hereinafter. For a better understanding of the nature and objects of the invention. reference should be had to the following detailed description and drawings, in which:

Figure 1 is a viewV in elevation, partly in cross section. of a bell-type furnace;

Fig. 2 is a vertical elevation, partly in section, of a pusher-type furnace for producing the insulating oxide iilms on ferrous metal;

Fig. 3 is a View in elevation, partly in section, of an annealing and film-forming pusher type furnace; and

Fig. 4 is a graph plotting resistance of a group of laminations on a probability graph with the vertical axis plotting ohms-per square centimeter on a logarithmic scale.

In accordance with the present invention, it has been found that various ferrous materials, particularly magnetic steel sheets and laminations, may be heat treated in a selected oxidizing atmosphere Within the temperature range between 400 C. and 570 C. to provide consistently a stable, highly insulative surface oxide film comprising almost entirely FeaOi and substantially free from iron, FeO and FezOa. The process can be applied to pure iron, low carbon steel, ironsilicon alloys having from 1/, silicon to 10% and more silicon, nickel-iron alloys having up to 50% nickel, alone, or nickel combined with cobalt, manganese, vanadium, molybdenum and other constituents of magnetic steels, and iron-cobalt alloys having up to 50 cobalt alone or with other constituents. The metal for treatment in the process will ordinarily contain more than 50% iron and be capable of oxidization in the temperature range of from 400 C. to 570 C. The time of heat treatment has been found to vary somewhat with the alloying constituents, the iron-cobalt and iron-nickel alloys require the longest times in a given atmosphere while silicon iron requires the shortest time.

It has been found that the ferrous metals to be provided with the stable insulating oxide films should be heat-treated in the temperature range of between 400 C. and 570 C. in an atmosphere composed of at least 20% by volume of at least one gas selected from the group consisting of water vapor and mixtures of water vapor with carbon dioxide. The mixtures of water vapor and carbon dioxide may have up to carbon dioxide. The ratio of the carbon dioxide to water should not exceed 9. 'I'he balance of the atmosphere may comprise one or more gases selected from the group consisting of hydrogen, carbon monoxide, nitrogen, and incidental impurities. Particularly good results have been obtained with an atmosphere of steam alone. If hydrogen is present, and some will unavoidably be developed by reaction of the water vapor with the iron, the volume ratio of water vapor to hydrogen should be greater than 0.25. If carbon dioxide is present along with carbon monoxide, the volume ratio of C02 to C0 should be greater than 1.2. In all the atmospheres, it has been found absolutely' necessary that the free oxygen be kept below l/2 of 1%. Free oxygen will cause formation of the undesirable red iron oxide which is loose and bulky, lacks satisfactory insulating characteristics and promotes further rusting instead of forming a protective coating.

If the ferrous metal to be provided with an insulating oxide fllm ls relatively free from red rust (FecOa) it may be heat treated in the range of from 400 C. to 570 C. in an atmosphere as indicated for a short period of time and it will acquire a consistently thin tightly adherent blueblack illm of highly insulative iron oxide comprising almost entirely Fea04. A suitable apparatus for carrying out the process may be any airtight heat treating furnace. Referring to Fig. 1 of the drawings, there is illustrated a bell-type furnace I suitable for the practice of the invention. The furnace comprises a base I2 upon which is placed an outer insulating shell I4 sealed with sand or molten metal I8 at the joint between the base. Heating elements I8 are disposed within the outer shell I4 for bringing the interior of the furnace to a temperature of from 400 C. to 570 C. Within the outer shell I4 is disposed an inner shell 20 enclosing an open ended cylinder 22 separated therefrom by a small space 32 for the flow of gases, as will be explained. A base support 24 and a perforated plate 26 are enclosed within the cylinder 22, a rotating fan 28 is disposed below the base support 24 for circulating gases through passages 30 up the annular space 32 and through the interior of the furnace where it is withdrawn through channels 34 in the base support 24. An atmosphere prepared as disclosed herein is introduced into the inner shell 20 by a suitable conduit 36 controlled by a valve 38 so as to envelope the ferrous metal charge 40 which is to be provided with the insulating oxide film. The charge 40, which may consist of coils of sheet metal, punched stacked laminations, stacks of at sheets, or even large massive bodies of oxidizable ferrous metal, is preferably supported on racks 42 to permit access of the atmosphere to the surfaces of the ferrous metal.

In operating the bell-type furnace I0, the charge 40 is placed upon the racks 42 and the inner shell 20 and outer shell I4 placed thereover, and then slowly heated. After the temperature has reached above 100 C. the interior may be purged with steam in orderto reduce the free oxygen to below l/2 of 1% by volume. It is desirable to complete the purging by the time a temperature of from 200 C. to 300 C. is reached. Thereafter the temperature may be brought up as rapidly as desired to within the range of 400 C. to 570 C. The time in the furnace will vary with the temperature, the type of laminations or sheet or other metal charge, the amount of oxide lm desired and the relative ease of oxidation of the ferrous metal. For electrical steel, a recommended cycle is 500 C. for four hours. The median ohmic resistance of 1% silicon iron produced at this time and temperature is better than 25 ohms per square centimeter per lamination where steam is the sole constituent of the atmosphere.

After the heat treatment has been carried out for a sufficient period of time, the charge may be cooled in the same atmosphere to a temperature of below 300 C. but not below 100 C. If considerable water vapor is present, the furnace may be purged when below 300 C., with an inert gas such for example as nitrogen, cracked ammonia, or the like in order to prevent the condensation of moisture on the metal. While the ferrous metal is well protected by the insulating oxide illm previously produced, moisture may condense between laminations and if the laminations were stored for several months or longer with the water between laminations, the formation of red rust might occur. The shells I4 and 20 may be removed from the furnace while the charge is at a temperature of from C. to 300 C., whereby the heat of the charge will prevent condensation of any water vapor. The temperature of the charge is low enough so that the oxygen in the atmosphere will no longer cause any undesirable results.

The process of the present invention may be carried out inY continuous type furnaces either chain operated or those operated by pusher mechanism. Referring to Fig. 2 of the drawings, there is illustrated a pusher type furnace suitable for carrying out the process of the present invention on a more or less continuous scale. The furnace |00 comprises a track |02 at one end of which is located a cylinder |04 and piston pusher mechanism |06 for pushing trays |08 carrying the ferrous metal to be provided with the stable insulating oxide film. The trays |08 containing ferrous metal in the form of laminations or the like are placed at the left-hand end of the track in position just ahead of the piston pusher |06. At stated intervals, the trays |08 containing a fresh charge of ferrous metal are pushed into a purgingchambe'r I|0 through a relatively close fitting door II2 which is adapted to effect a close seal with the edge of the trays. If desired, sliding doors may be employed to close the opening |I2 to furnish a more effective seal. The purging chamber I0 is furnished with a purging gas from a conduit ||4 at a pressure above atmospheric to drive off the oxygen present in the charging trays |08. A variety of atmospheres are suitable for this purpose including nitrogen, cracked ammonia, or a combusted atmosphere of low oxygen and low water vapor content and the like.

When the piston pusher |06 is retracted, another tray |08 is placed on the track |02 and after a timed interval the second tray pushes the preceding tray from the purging chamber into an oxidizing chamber I I6. In the oxidizing chamber IIB, the temperature of the charging tray |08 is brought up to from 400 C. to 570 C. Through a conduit ||8 there is supplied to the chamber |I6 an atmosphere containing less than 1/2 of 1% free oxygen, preferably with no free oxygen, and having at least 20% by volume of at least one oxidizing gas selected from the group consisting of water vapor and mixtures of water vapor with carbon dioxide. The atmosphere may be composed entirely of steam or steam mixed with carbon dioxide and it may contain hydrogen, nitrogen, carbon monoxide and incidental impurities. The ratio of water vapor to hydrogen must be maintained above 0.25 and the ratio of carbon dioxide to carbon monoxide must be maintained above a value of 1.2. The oxidizing chamber ||6 should be heated to such a temperature and should be of such a length that the contents of each tray are exposed to the atmosphere for a period of time sufficient to produce a desirable thickness of the insulating iron oxide film. For magnetic laminations, a thickness of oxide of from 0.01 to 0.5 mil thickness has been found to be satisfactory. Oxide films of this thickness may be secured within a period of from minutes to four hours depending on the temperature and the composition oi' the atmosphere.

The trays I 08 are pushed from the chamber I I0 into a cooling chamber |20, provided with the same atmosphere by a conduit |22 as contained in chamber ||6 or an inert gas or other atmosphere. not deleterious to the oxide film on the charge. 'Ihe temperature of the trays with their contents is reduced to below 300 C. and then the trays are moved into an exit chamber |24 provided with a relatively inert atmosphere from conduit |25 to prevent diffusion of atmospheric oxygen into the cooling chamber and the oxidizing chamber I5. A pressure slightly above atmospheric is desirable in the exit chamber |24. A suitable atmosphere for the exit chamber is nitrogen or a combusted gas free from oxygen. The trays are finally pushed into the outlet position |28.

The process of this invention has been found to produce consistently uniform results when operated within the limits disclosed. Laminations of a thickness of 0.0185 inch of 1% silicon iron, heat-treated in an atmosphere composed entirely of steam for four hours at a temperature of 500 C. acquired an oxide lm having a median resistance of 37 ohms per square centimeter. The same laminations heat-treated in an atmosphere composed of 90% hydrogen, 10% water vapor had vzo a median resistance of less than one ohm per square `centimeter, a good portion of the resistance in this latter instance being attributed to temper oxide lms which formed when the laminations were exposed to the atmosphere. The

same laminations heat-treated in an atmosphere of 50% hydrogen, 50% water vapor had a median resistance of 28 ohms per square centimeter. Again when heat-treated in an atmosphere composed of 40% water vapor, 15% nitrogen and 45% hydrogen the median resistance was ohms per square centimeter.

The following table gives further examples of the invention:

In many cases, the ferrous metal which it is desired to provide with the insulating oxide lms disclosed herein may carry on its surfaces undesirable iron oxides such as FezOx which is red and bulky or may carry other oxide coatings which are non-electrically insulative due to the presence of iron and the like. Thus steel during its manufacture usually is heat-treated to a temperature above 570 C. in an oxidizing atmosphere whereby iron oxide (FeO) has been formed and on cooling below 570 C., the iron oxide will be decomposed to produce a mixture of free iron particles dispersed in an oxide film. X-ray diraction studies have disclosed that this type o f film has large amounts of iron present and insulation tests show it to have a low resistance, ordinarily one ohm and less per square centimeter. In acface oxides first may be eliminated and then replaced with the highly insulative adherent oxides of this invention. Furthermore, in this process a stress relieving treatment for magnetic laminations or sheet or the like may be effected so that a plurality of desirable features are obtained in a single integrated operation. 'I'he process may be carried out in any suitable heat-treating furnace in which a. controlled atmosphere may he applied and replaced with another atmosphere.

Referring to Fig. 3 of the drawings, there is illustrated a furnace |30 suitable for stress relieving ferrous metal and reducing any surface oxides on the ferrous metal in a single operation and thereafter producing on the surface of the heat-treated ferrous metal the oxide coatings according to the process of the present invention. The furnace |30 is a pusher-type furnace and is provided with a track |32 at the left end of which the cylinder |34 and pusher piston |36 push trays |38 from left to right through the various zones in the furnace. The trays |38 pass into a purging chamber |40 through a sealing door |42. A conduit |44 supplies a purging atmosphere to the chamber |40 to drive off the free oxygen entrance in the tray |38, the atmospheres may be thus indicated above for purging chamber ||0 in Fig. 2. The trays |38 are then introduced into an annealing chamber where they are heated to a temperature of the order of from '100 C. to 950 C. A reducing atmosphere is introduced through the conduit |52 for reducing any surface oxides to metallic iron and to decarburize the metal is so desired. Suitable atmospheres are hydrogen, cracked ammonia, and combusted gases. If stress relieving of the metal is desired, the time required in the furnace is adequate to insure all the surface oxides being reduced. Ordinarily two to twelve hours is adequate to stress relieve the metal and to reduce iron oxides. Thereafter the trays are pushed into the zone |54 separated by baille |55 from the annealing chamber |50 containing the reducing atmosphere where the trays and their contents are reduced in temperature to 570 C. to 400 C. A conduit |58 may furnish an atmosphere similar to that in chamber |50, or the atmosphere admitted from chamber |50 under the baille |56.

In order to prevent diffusion of the oxidizing atmosphere from succeeding portions of the furnace into the deoxidizing or reducing chambers, the trays |38 are pushed into a transfer chamber containing a reducing gas which may conveniently be the same gas employed in an annealing furnace or other reducing gas. The reducing gas is supplied by means of a conduit |62 and is maintained at a slightly higher pressure than the succeeding oxidizing chamber atmospheres so that the diffusion of the gas is into the oxidizing chambers rather than into the atmosphere in the annealing chamber |50 and the cooling .chamber |54. Furthermore, the transfer chamber |50 permits the metal in the trays |38 to reach a uniform temperature within the range of 400 C. to 570 C.

The transfer chamber |60 is provided with doors |64 and |05 to seal oi the annealing zone chambers from the oxidizing portion of the furnace. In order to push trays therethrough there is provided a hook'l68 pivotally mounted on a vertical arm |12 and urged upwardly by a spring |10 to catch behind trays |38. The vertical arm |12 is mounted on a piston rod |14 for movement cordance with this invention the undesirable suru in accordance with a piston |13 mounted in a cylinder |18. In operation the piston. |18 is at its extreme left position when the door |64 opens and permits a tray |38 to be pushed into the transfer chamber |60 by pusher |36. After the tray passes over the hook |68 by pivoting it down, the hook rises to catch its left-hand end. The door |64 closes and door |66 opens and the piston |16 pushes the tray |38 into the oxidizing zone and the door |66 closes.

The trays are pushed by hook |68 into an oxidizing chamber |80 containing an atmosphere of water vapor or carbon dioxide and water vapor in which these gases constitute over 20% by v01- ume of the atmosphere and the remainder are relatively non-oxidizing gases such as nitrogen or carbon monoxide or hydrogen proportioned so that the ratio of water vapor to hydrogen is greater than 0.25 and the ratio of carbon dioxide to carbon monoxide is greater than 1.2.

The atmosphere is provided through the inlet |82. Suitable circulating means are provided as is well known in the art to insure contacting of the ferrous metal with the atmosphere. After the ferrous metal in trays |36 has remained in chamber |80 for a period of time long enough to provide thereon the desired insulating oxide lm composed substantially entirely of Fe304, the trays are pushed into the cooling chamber |84 where they are cooled to a temperature of above 100 C. and below 300 C. in the presence of the oxidizing atmosphere which is supplied by a pipe |86. After cooling to this temperature range, the trays are pushed into the exit chamber |88 provided with a relatively inert atmosphere, which may be nitrogen, through a conduit |90, to further cool the metal being treated and to prevent the diffusion of the atmospheric oxygen into the furnace. Thence the trays are pushed out into the open air for handling at position |92 at the extreme right-hand of the track |32.

It will be appreciated that the construction and mode of operation of the furnaces in Figs. 1, 2 and 3 is not critical to the process but that they may be modified in accordance with requirements. For example, the furnaces may be heated by resistor elements or by suitable gas or other heating appliances, and the closures between the various chambers may be operated in any suitable manner. The precise time cycles must be determined in accordance with the particular requirements of the ferrous metal being treated. Thus the annealing cycle time in a furnace operating as in Fig. 3 may be much shorter than the oxidizing cycle or again much longer in order to accomplish satisfactory stress relieving and oxide reduction. In some cases, sheet ferrous metal may pass as a continuous strip from zone to zone. Assembled laminations in the form of magnetic rotors, stators, relay cores, and magnetic cores in general may be passed through. the furnace in order to relieve the punching strains as well as the strains induced by rivets, bolts or other core assembly fastening means.

The relationship between time and temperature required to produce an insulating oxide film of a given resistance using a given atmosphere for any particular ferrous metal has been found to vary exponentially as follows. The time t is related to temperature T in accordance with the following equation:

where A and B are constants and e is the base of natural logarithm. As an example, 1% silicon iron oxidized by 100% water vapor atmosphere followed the following time temperature equation to produce a film having a median resistance of 10 ohms per square centimeter:

where t is the time in minutes, e is the base of natural logarithms and T is the absolute temperature in degrees centigrade.

Because of the difiiculties in maintaining a temperature of precisely 570 C. or slightly under, in practice the maximum temperature in the oxidizing chambers is set at 550 C. Further in order to secure the oxide film in the least practical time, the lowest temperature should be 450 C. Within the range of 450 C. to 550 C. excellent oxide films will be obtained consistently in relatively short heat-treating times.

It will be appreciated that While the major proportion of the films produced in accordance with the process of this invention are constituted by the iron oxide FesOl, there may be present the oxides of the other constituent of the ferrous metal such for example as silica. In order to avoid an excess of the other metal oxides, the ferrous metal being treated should contain a major proportion of iron, that is, at least 50% of the ferrous metal must be composed of Fe.

In order to show the improvment attainable by the practice of the present invention, two batches of laminations of the same silicon iron having 1% silicon and 0.0185 inch thick were processed separately. One batch was heattreated for 4 hours at a temperature of 725 C. in a combusted atmosphere containing 15% H2O, 12% CO2, 5.4% CO, 3% Hz and the balance nitrogen. The ohmic resistance of each lamination was measured and plotted with the results as illustrated in curve A in Figure 4 on a probability graph. The median resistance was 0.11 ohm per square centimeter. An all steam atmosphere could not be used at this temperature since the stack of laminations would be stuck together and would scale excessively. The second batch of laminations was heat-treated for 4 hours at 500 C. and the ohmic resistance of the lamina'- tions was determined and plotted as curve B in Fig. 4. The median resistance was 50 ohms per square centimeter. X-ray diffraction studies of the oxide lms indicated that the material from which curve A was plotted contained substantial amounts of free iron while the oxide films present on the samples from which curve B was made were composed almost entirely of FegOi and had no free iron present.

Since certain obvious changes may be made in the above process, and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all material contained in the above description or taken in connection with the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. In the method of heat-treating ferrous metal laminations having surface oxides present thereon to relieve stresses and to provide'a stable, highly insulative oxide surface film thereon, the steps comprising heating the laminations to a temperature above 570 C. in a reducing atmosphere for a length of time to reduce any surface ferrous oxides that may be present on the laminations and simultaneously to relieve stresses, cooling the laminations to a temperature of from 450 C. to 550 C. while in a reducing H2O and CO2, and the balance composed of at least, one gas selected from the group consisting of H2, CO, N2, the volume ratio of H2O to H2 being greater than 0.25, the ratio of CO2 to CO being greater than 1.2, and the free oxygen being below l/2 of 1%, the heating being conducted for a time suillcient to producea highly insulating lm of Fe304.

2. In the method of heat-treating .ferrous metal laminations having surface oxides present thereon to relieve stresses and to provide a stable, highly insulative oxide surface lm thereof, the steps comprising heating the laminations to a temperature above 570 C, in a reducing atmosphere for a length of time to reduce any surface ferrous oxides that may be present on the laminations and simultaneously to relieve stresses, cooling the laminations to a temperature of from 450 C`. to 550 C. while in a reducing gas atmosphere, applying to the laminations while in the range of temperatures of from 450 C. to 550 C. an atmosphere composed of at least 20% by volume of at least one gas selected from the group consisting or H20 and mixtures of H2O and CO2, and the balance composed of at least one gas selected from the group consisting of H2, CO, N2. the volume ratio of H2O to Hz being greater than 0.25, the ratio of CO2 to CO being greater than 1.2, and the free oxygen being below 1/2 of 1%, the heating being conducted ior a. time sumcient to produce a highly insulating fllmof Fea04, and cooling to a temperature below 300 C. and above 100 C. in this atmosphere before exposing to air.

PAUL L. SCHMIDT. LAWRENCE R. HILL.

REFERENCES CITED The following references are of record ln the le o! this patent:

UNITED STATES PATENTS Number Name Date 182,148 Baril L Sept. 12, 1876 234,524 Bower et al. Nov. 16, 1880 331,104 Winslow Nov, 24, 1885 1,056,627 Carnahan et al. Mar. 18, 1913 1,115,281 Carnahan et al. Oct. 27, 1914 1,346,473 Swan ---1 July 13. 1920 2,236,728 Given Apr. 1, 1941 2,269,943 Kiser Jan. 13, 1942 2,333,936 Johnson Nov. 9, 1943 

1. IN THE METHOD OF HEAT-TREATING FERROUS METAL LAMINATIONS HAVING SURFACE OXIDES PRESENT THEREON TO RELIEVE STRESSES AND TO PROVIDE A STABLE, HIGHLY INSULATIVE OXIDE SURFACE FILM THEREON, THE STEPS COMPRISING HEATING THE LAMINATIONS TO A TEMPERATURE ABOVE 570* C. IN A REDUCING ATMOSPHERE FOR A LENGTH OF TIME TO REDUCE ANY SURFACE FERROUS OXIDES THAT MAY BE PRESENT ON THE LAMINATIONS AND SIMULTANEOUSLY TO RELIEVE STRESSES, COOLING THE LAMINATIONS TO A TEMPERATURE OF FROM 450* C. TO 550* C. WHILE IN A REDUCING GAS ATMOSPHERE, APPLYING TO THE LAMINATIONS WHILE IN THE RANGE OF TEMPERATURES OF FROM 450* C. TO 550* C. AN ATMOSPHERE COMPOSED OF AT LEAST 20% BY VOLUME OF AT LEAST ONE GAS SELECTED FROM THE GROUP CONSISTING OF H2O AND MIXTURES OF H2O AND CO2, AND THE BALANCE COMPOSED OF AT LEAST ONE GAS SELECTED FROM THE GROUP CONSISTING OF H2, CO, N2, THE VOLUME RATIO OF H2O TO H2 BEING GREATER THAN 0.25, THE RATIO OF CO2 TO CO BEING GREATER THAN 1.2, AND THE FREE OXYGEN BEING BELOW 1/2 OF 1%, THE HEATING BEING CONDUCTED FOR A TIME SUFFICIENT TO PRODUCE A HIGHLY INSULATING FILM OF FE3O4. 